Which saint's cross is represented by the red diagonal lines on the Union Jack?

The red diagonal lines on the Union Jack are conventionally identified as Saint Patrick’s Saltire, or Saint Patrick’s Cross. They represent Ireland in the flag’s combined design alongside the crosses of Saint George and Saint Andrew.

The saltire is a red X-shaped cross on a white field. The Order of Saint Patrick adopted it as an emblem in 1783, and it was incorporated into the Union Flag after the Acts of Union 1800 joined Great Britain and Ireland. The present Union Jack dates from 1 January 1801.

The attribution is historically complicated. The saltire’s origin is disputed, and some historians connect the design to the FitzGerald dynasty rather than to Patrick himself. Irish nationalists also criticized it as a British invention. Those debates do not change the standard quiz identification: the red saltire is called Saint Patrick’s Cross, while Saint Andrew’s Cross is the white diagonal saltire and Saint George’s Cross is the red upright cross.